Update book.

docs/en/src/apollo_federation.md

@@ -51,3 +51,27 @@ impl Query {
     Use `id` and `username` to find an `User` object, the keys for `User` are `id` and `username`.
 
 For a complete example, refer to: <https://github.com/async-graphql/examples/tree/master/federation>.
+
+## Defining a compound primary key
+
+A single primary key can consist of multiple fields, and even nested fields, you can use `InputObject` to implements a nested primary key.
+
+In the following example, the primary key of the `User` object is `key { a b }`.
+
+```rust
+#[derive(InputObject)]
+struct NestedKey {
+  a: i32,
+  b: i32,
+}
+
+struct Query;
+
+#[Object]
+impl Query {
+  #[entity]
+  async fn find_user_by_key(&self, key: NestedKey) -> User {
+    User { ... }
+  }
+}
+```

docs/zh-CN/src/apollo_federation.md

@@ -51,3 +51,27 @@ impl Query {
     使用`id`和`username`查找`User`对象，`User`对象的key是`id`和`username`。
 
 完整的例子请参考https://github.com/async-graphql/examples/tree/master/federation
+
+## 定义复合主键
+
+一个主键可以包含多个字段，什么包含嵌套字段，你可以用`InputObject`来实现一个嵌套字段的Key类型。
+
+下面的例子中`User`对象的主键是`key { a b }`。
+
+```rust
+#[derive(InputObject)]
+struct NestedKey {
+  a: i32,
+  b: i32,
+}
+
+struct Query;
+
+#[Object]
+impl Query {
+  #[entity]
+  async fn find_user_by_key(&self, key: NestedKey) -> User {
+    User { ... }
+  }
+}
+```